[R-390] Isolation Transformers, GCFI and Balanced Power

[Roy Morgan]

On Apr 15, 2014, at 6:10 AM, Ken Harpur <igloo99nz at yahoo.co.nz> wrote:
> I'm having issues with the line filters in my R-390As tripping the GCFI (doesn't everyone now?)…

Those of us who live in older houses (read: not up to code for GFI outlets) don’t have the problem, but some may well have the “hot chassis” problem, where about 60 volts appears from the chassis to ground, IF the chassis is not grounded.  The reason is that the bypass caps in the filter are creating a voltage divider, and only SOME times because there is a leaky cap in there.

One common way to overcome the trouble is to remove the original filter, and make a plate that will hold a  modern line filter nicely in the space.  (Do not hope to buy a Greenlee punch that makes the hole - they are priced at over $400.00)  Then use the common computer style line cords.  (Are they the same in NZ?)

> ... One filter is bad so it has been removed, awaiting a replacement.

The filters are kind of a mess, and difficult to get apart.  Note that the replacement may not be better than the intermittent ones.  

> ... I came across a post saying that the filters could be left intact and the radios plugged into an Isolation transformer. The Isolation transformer will prevent the GCFI from tripping…

Sounds like a fine solution.  Don’t forget to ground the radio well in any case.  Even if there IS some safety ground wire current, I think that the GFI devices won’t sense it.  They sense imbalance in the line and neutral wires, and make the assumption that a person is making the connection between the line and ground (maybe some other ground such as a faucet or puddle, not necessarily the circuit safety ground wire).

> 
> Hmm...yep seems fair enough and I have a 230 to 115V transformer here so it's do-able. I could re-wire the power supply on one radio to 115V and try it out.

Isn’t there a switch on the power supply you move?  ‘Can’t remember.

> So what do you do to mitigate noise on your line supply, if anything?

One possibility is to use a voltage regulating transformer - common name “Sola”, but made also by other companies.  Normally these things have an isolated secondary, though if there’s a three wire grounded outlet socket, the safety ground will likely be connected through to the case and the ground on the supply cord.

Added benefits to regulator transformers may well be:
 - Noise reduction.  The thing may have a shield between the output winding and the input winding(s).  Manufacturer’s specs indicate noise isolation figures of 40 db.*  If the secondary is isolated, then there is no direct connection as there is in most variable voltage transformers (“Variac”).  Isolated variable transformers do exist, but I’ve never seen one.
- Over current protection.  “What?  I won’t need a fuse?!”  It seems that the nature of the voltage regulation circuit is that with over-current of about 160 percent of rating*, the voltage collapses.  There is no harm to the device, and it returns to  normal once the over current is removed.  So, if you have an R-390 drawing an amp (as a guess) and your “sola” is rated at 90 VA or a bit more, you have built in protection.  I have a Sola rated at 60 va, so I don’t expect that one to run an R-390.  The ideal one would be the Sola 90 va or 120 VA ones.

*Sola CVS Hardwired Series
See:  <http://www.solahevidutysales.com/cvs_hardwired_series_power_conditioner.htm>


The Non-Benefits to the sola include these:
 - they are somewhat heavy  (65 pounds for the 1kw size)
 - with little power drawn, they still make a lot of heat
 - they are noisy, especially the bigger ones
 - If you come across one that’s not working - it’s most likely the capacitor(s) inside, and that’s easy to replace.

One further note:  The Sola brand transformers are guaranteed for 10 years.  In a world full of 90 day guarantees, this means that the company thinks they almost never fail.  In practical use, they really do almost never fail.  So if you see one at a hamfest that looks really old and the price is modest, go for it.

An UNtested possibility:  If your sola device puts out modern line voltage, it MAY be possible to add a few-turn winding onto the thing to be wired in voltage opposition to the output connection.  This will make it into a line bucking voltage regulating isolation shack warming device.  

Who could want more?

Roy

Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958